Inductance unit



Feb. 11, 1958 J. J. HOPKINS ET AL 2,823,361

INDUCTANCE UNIT Filed July 31, 1946 FIG. 2.

INVENTORS JOHN J HOPKINS PHIL lP RUPi/ICK BY I ATTORNEY United States Patent INDUCTAN CE UNIT John J. Hopkins, Silver Spring, Md., and Philip Rudnick, Nashville, Tenn., assignors to the United States of America as represented by the Secretary of the Navy Application July 31, 1946, Serial No. 687,436

3 Claims. (Cl. 336-69) This invention relates to an inductance unit or choke coil and has particular relation to a choke coil which will have characteristics such as to be capable of being designed to resonate at frequencies below a certain maximum; that is, the inductance and capacitance will be such that their product does not fall below a predetermined value.

When inductance units are made of physically small size and of fine Wire, difiiculties arise in maintaining a relatively high LC product in that the surface of the wire constituting the winding becomes small, and hence its distributed capacitance likewise drops.

It has now been found that by winding such coils on forms or cores made of a particular dielectric medium, the capacitance may be increased approximately in proportion to the dielectric constant of the medium. This result has been attained by employing a barium titanate compound as the major ingredient of the composition of which the core is molded. It is preferred to employ a barium titanate of high dielectric constant disclosed in U. S. Patent 2,377,910 and now available commercially, which has a dielectric constant approaching 1400, although other barium titanate compounds are suitable. Cores or forms may be made by mixing this substance with a suitable binder, such as a synthetic or other resin.

An example of a choke embodying the invention is illustrated in the accompanying drawing in which:

Fig. 1 is an elevation of the choke, and

Fig. 2 is an end view.

Referring to the drawing, the inductance core is molded in cylindrical form with tinned copper wires or rods 11 and 13 embedded therein at the ends of the core 10. The rods 11 and 13 are so positioned that their inner ends are in close proximity to each other for the purpose of adding to the distributed capacity of the inductance unit. An enameled copper wire coil 12 is wound around the core 10 with a suitable number of turns, the terminals 14 and 15 of the coil 12 being secured about the rods 11 and 13, as by means of winding or soldering. The completed inductance unit is then coated with a suitable protective coating such as a stoving varnish consisting of a suitable solution of a condensation product of the glycerol-phthalic anhydride type after being oven dried to remove moisture from the unit. In practice, inductance units having a length of .5 inch and a diameter of .138 inch have been made. Such units were wound with 37 turns of No. 30 B. & S. gauge enameled copper wire, the rods 11 and 13 being of N0. 22 tinned copper wire. The rods 11 and 13 were positioned in the manner indicated in Fig. i so as to be internally separated at their inner ends by a distance of 0.042 inch. Each of these units resonated at the low frequency of megacycles.

A preferred core composition consists by weight of about 93% of a suitable barium titanate mixed with 7% of a thermosetting synthetic intermediate of a resin of the phenol-formaldehyde type.

Barium titanate in comminuted form is mixed in the proper proportion with the phenol-formaldehyde resin intermediate, molded and baked to transform the latter into its final condensed polymerized form.

Because of the extremely high dielectric value of the core a relatively high capacitance may be produced with comparatively small conductors, so that even the capacitance due solely to the terminal rods 11 and 13, for example, may be extremely high.

While 93% has been given as the preferred amount of titanate, it will be understood that useful results may be obtained even with slight variations in this proportion. More than 93% serves to increase the dielectric constant of the resulting core, but decreases its mechanical strength and other properties whereas considerably more than 7% of the phenol-formaldehyde resin enhances its mechanical properties at the expense of a lower dielectric constant. It has been found that the useful upper limit of resin is about 10%.

What is claimed is:

1. In an inductance unit, a dielectric core comprising by weight a major proportion of barium titanate and a minor proportion of binder resin, a conductive winding mounted thereon, conductive terminals connected to the leads of said winding and at least partially embedded in said core with their inner ends spaced apart a distance merely sufiicient to prevent accidental contact with each other whereby the capacitance between the embedded portions augments the distributed capacitance of the unit.

2. The inductance unit set forth in claim 1 wherein the dielectric core comprises by weight not less than approximately 90% barium titanate.

3. The inductance unit set forth in claim 1 wherein the dielectric core comprises by weight approximately 93% barium titanate.

References Cited in the file of this patent UNITED STATES PATENTS 2,106,039 Saft'ord Ian. 18, 1938 2,355,611 Veley Aug. 15, 1944 2,377,910 Wainer June 12, 1945 

